I tried RL-26 in my Roberts, burned dirty, left granules of unburned powder in the chamber, couldn't chamber the next round. Shook'em out, chambered easily.
DF
If you want to try again switch to a magnum primer and use a full bore load and a heavy bullet. Many slow powders are like burning charcoal until you hit the pressure level they were designed for. I have seen this with H870 consistently. A tight neck can help sometimes too. I am not seeing too much crud with R26 and the similar 6mm AI. Bore looks a little dirty but it knocks out with one patch. Some are theorizing the anti coppering agency is what your seeing and is not carbon build up. YMMV
Unburned powder in the chamber can be a sign you are not generating enough pressure for the case to seal itself. Put the hammer down on those loads and they may come around.
